Benzene, toluene, and xylenes (BTX) are important aromatic hydrocarbons, for which the worldwide demand is steadily increasing. The demand for xylenes, particularly paraxylene, has increased in proportion to the increase in demand for polyester fibers and film and typically grows at a rate of 5-7% per year. Benzene is a highly valuable product for use as a chemical raw material. Toluene is also a valuable petrochemical for use as a solvent and an intermediate in chemical manufacturing processes and as a high octane gasoline component. However, in many modern aromatic complexes, some or all of the benzene and/or toluene is converted to further xylenes by either transalkylation or methylation or a combination thereof.
A major source of benzene, toluene, and xylenes (BTX) is catalytic reformate, which is produced by contacting petroleum naphtha with a hydrogenation/dehydrogenation catalyst on a support. The resulting reformate is a complex mixture of paraffins and the desired C6 to C8 aromatics, in addition to a significant quantity of heavier aromatic hydrocarbons. After removing the light (C5−) paraffinic components, the remainder of reformate is normally separated into C7−, C8 and C9+-containing fractions using a plurality of distillation steps. Liquid-liquid or extractive distillation is then typically required to remove non-aromatic co-boiling compounds from the C7−-containing fraction before the benzene can be recovered to leave a toluene-rich fraction which is generally used to produce additional C8 aromatics by either methylation or disproportionation. The C8-containing fraction is fed to a xylene production loop where paraxylene is recovered, generally by adsorption or crystallization, and the resultant paraxylene-depleted stream is subjected to catalytic conversion to isomerize the xylenes back towards equilibrium distribution and to reduce the level of ethylbenzene that would otherwise build up in the xylene production loop. Transalkylation may also be added to convert at least part of the C9+-containing fraction to additional xylenes by reaction with some of the benzene and/or toluene recovered from the C7−-containing fraction.
While effective, these technologies are capital and variable cost intensive, and there is a general need in the industry to reduce or eliminate costs associated with upgrading the non-xylene aromatic molecules in traditional aromatic feedstocks. Furthermore, with most traditional aromatic feedstocks, the molar ratio of methyl groups to aryl groups, is less than the optimal range (1.8:1-2.2:1 on a molar basis) for maximizing the yield of xylene product per ton of feedstock. There is therefore significant interest in the development of low cost and efficient approaches to the problem of increasing xylene yields from traditional aromatics feedstocks.
U.S. Pat. No. 6,504,072 discloses a process for the selective production of paraxylene which comprises reacting toluene with methanol under alkylation conditions in the presence of a catalyst comprising a porous crystalline material having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa). The porous crystalline material is preferably a medium-pore zeolite, particularly ZSM-5, which has been severely steamed at a temperature of at least 950° C. The alkylation conditions include a temperature between about 500 and 700° C., a pressure of between about 1 atmosphere and 1000 psig (100 and 7000 kPa), a weight hourly space velocity between about 0.5 and about 1000 and a molar ratio of toluene to methanol of at least about 0.2.
In addition, U.S. Pat. No. 6,642,426 discloses a process for alkylating an aromatic hydrocarbon reactant, especially toluene, with an alkylating reagent comprising methanol to produce an alkylated aromatic product, comprising: introducing the aromatic hydrocarbon reactant into a reactor system at a first location, wherein the reactor system includes a fluidized bed reaction zone comprising a temperature of 500 to 700° C. and an operating bed density of about 300 to 600 kg/m3, for producing the alkylated aromatic product; introducing a plurality of streams of said alkylating reactant directly into said fluidized bed reaction zone at positions spaced apart in the direction of flow of the aromatic hydrocarbon reactant, at least one of said streams being introduced at a second location downstream from the first location; and recovering the alkylate aromatic product, produced by reaction of the aromatic reactant and the alkylating reagent, from the reactor system. The preferred catalyst is ZSM-5 which has been selectivated by high temperature steaming.
As exemplified by the U.S. patents discussed above, current processes for the alkylation of benzene and/or toluene with methanol are conducted at high temperatures, i.e., between 500 to 700° C., in the presence of a medium pore size zeolite, particularly ZSM-5. This results in a number of problems, particularly in that catalyst life per cycle is relatively short and so frequent regeneration of the catalyst is required. In addition, the existing processes typically result in significant quantities of methanol being converted to ethylene and other light olefins which reduces the yield of desirable products, such as xylenes, and increases recovery costs.
There is therefore a need for an improved process for the alkylation of benzene and/or toluene with methanol (or dimethyl ether), which increases catalyst cycle life and reduces gas make as well as facilitates integration of the process into a paraxylene production complex.